1. Field of the Invention
The present invention relates to a clock signal generator, and more particularly, to a clock signal generator for a Universal Serial Bus (USB) device.
2. Description of the Related Art
The USB interface was developed to standardize an interface for connecting a computer and peripheral devices such as a mouse, a printer, a modem, scanner, and speakers, etc. The USB interface is a kind of serial port that was developed by the consortium of PC manufactures such as Intel, Compaq, Microsoft, Philips, IBM, and NEC, which attracted public attention with the launch of Windows 98. This disclosure incorporates by reference the contents of the USB Specification Revision 2.0 dated Apr. 27, 2000.
Because the use of the USB interface can avoid the costly installation of complex adapters, most standard PC devices are equipped with USB ports. Currently, the USB interface is widely used to transfer data from mobile phones or digital cameras to personal computers (PCs). A first generation (USART) serial port can only support a data transfer rate of at most 100 Kbps. On the other hand, a full-speed USB port can support a data transfer rate of up to 12 Mops. In addition, the full-speed USB can be connected to up to 127 devices in a hub or chain configuration. Furthermore, a full-speed USB device can be connected (hot swapped) and recognized even during the use of a PC. Moreover, many full-speed USB peripheral devices requires no external power.
A smart card is a plastic, card having the same size as a credit card, and includes an integrated circuit (IC) chip that includes a microprocessor, a card operating system (software), a security module, and a memory (RAM, ROM, EEPROM, Flash, etc.) and thus can perform a specific transaction. A typical smart card includes a plastic card having the same size and thickness as a credit card and a 0.5 mm thick semiconductor chip installed within the plastic card in a chip-on-board (COB) configuration.
Smart cards have rapidly emerged as the next-generation multimedia information medium because it has a higher stability than a magnetic-stripe card, has no possibility of data being erased, and has higher security.
The recent increase in application for smart card, has increased the need for rapid communication between a computer system and a smart card. For example, for user authentication of a computer system using a smart card, information in the smart card must be transferred to the computer system. Conventional smart card readers are relatively expensive, relatively slow serial devices. What is therefore desired is communication between a smart card and a computer system using a USB interface. However, a smart card's thin dimension does not allow for the inclusion of a crystal oscillator typically employed in the USB port of a peripheral device.
The USB specification requires that a data transfer rate be maintained to a predetermined accuracy. For example, an accuracy of ±0.25% is required in a full-speed mode (12 Mbps) of USB 1.1. This accuracy can be achieved through clock accuracy. Because a USB bus includes no clock line, each device must be able to internally generate a clock signal itself. If a device cannot generate an accurate clock signal, the device is not USB-compatible.
In general, an RC oscillator has an accuracy of ±10% and is insufficient. A clock signal generated by a more accurate oscillator circuit typically has an accuracy of ±3%. To enhance the accuracy of an oscillator circuit, a crystal is connected to a semiconductor chip. However, in this case, the semiconductor chip must have at least two additional pins for connecting the crystal to the outside of the semiconductor chip, which causes additional costs. Furthermore, the crystal is relatively large and thus cannot be used in chip cards such as smart cards and memory cards.